Liz, to me it sounds like a very good deal. Trained horse, up to 4 days rides, and $150 is not bad at all. How experienced are you? If you are more of a beginner I'd suggest to ride this horse first couple times to see if it's a good match and not too much of horse. And if you both click and you decide on lease be sure to write/sign the contract with the owner.
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."